Cooperative Communication Protocols in Wireless Networks: Performance Analysis and Optimum Power Allocation |
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Authors: | Weifeng Su Ahmed K Sadek K J Ray Liu |
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Affiliation: | (1) Department of Electrical Engineering, State University of New York (SUNY) at Buffalo, Buffalo, NY 14260, USA;(2) Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742, USA |
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Abstract: | In this paper, symbol-error-rate (SER) performance analysis and optimum power allocation are provided for uncoded cooperative
communications in wireless networks with either decode-and-forward (DF) or amplify-and-forward (AF) cooperation protocol,
in which source and relay send information to destination through orthogonal channels. In case of the DF cooperation systems,
closed-form SER formulation is provided for uncoded cooperation systems with PSK and QAM signals. Moreover, an SER upper bound
as well as an approximation are established to show the asymptotic performance of the DF cooperation systems, where the SER
approximation is asymptotically tight at high signal-to-noise ratio (SNR). Based on the asymptotically tight SER approximation,
an optimum power allocation is determined for the DF cooperation systems. In case of the AF cooperation systems, we obtain
at first a simple closed-form moment generating function (MGF) expression for the harmonic mean to avoid the hypergeometric
functions as commonly used in the literature. By taking advantage of the simple MGF expression, we obtain a closed-form SER
performance analysis for the AF cooperation systems with PSK and QAM signals. Moreover, an SER approximation is also established
which is asymptotically tight at high SNR. Based on the asymptotically tight SER approximation, an optimum power allocation
is determined for the AF cooperation systems. In both the DF and AF cooperation systems, it turns out that an equal power
strategy is good, but in general not optimum in cooperative communications. The optimum power allocation depends on the channel
link quality. An interesting result is that in case that all channel links are available, the optimum power allocation does
not depend on the direct link between source and destination, it depends only on the channel links related to the relay. Finally,
we compare the performance of the cooperation systems with either DF or AF protocol. It is shown that the performance of a
systems with the DF cooperation protocol is better than that with the AF protocol. However, the performance gain varies with
different modulation types and channel conditions, and the gain is limited. For example, in case of BPSK modulation, the performance
gain cannot be larger than 2.4 dB; and for QPSK modulation, it cannot be larger than 1.2 dB. Extensive simulation results
are provided to validate the theoretical analysis. |
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Keywords: | Cooperative communications Amplify-and-forward protocol Decode-and-forward protocol Symbol error rate Performance analysis Optimum power allocation Wireless networks |
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